Installation of e.g. flow lines at increased water depths introduces new challenges caused by the increased weight of the product and the corresponding increase in tension of the material during the laying operation. This will have impact on the risk of damaging the product, on the safety handling during the operation, and also on the cost of the product and of the pipe laying operation. In particular, the increasing water depths necessitate general upgrades of the existing operating parameters and equipments. Examples are increased squeeze pressure around flow lines, increased number of tensioners, higher vertical lay system (VLS) towers and wider ships due to higher stability requirements.
Methods and apparatuses for laying flexible pipes, cables, etc. on a seabed are well known. For example, WO 03/004915 (Stockstill) and U.S. Pat. No. 5,346,333 (Maloberti, et al.) both describe conventional VLS. In a traditional VLS, the flexible pipe is held by tensioners, often in series and having synchronized motions in order to control the laying process. However, such conventional VLS necessitate the handling of high loads from the deep water flow lines involving the need for a high number of tensioners making the overall system inter alia large, cumbersome and costly with high demands of continuous maintenance.
The state of the art also includes WO 2012/044179 A1 (Haugen, et al.), which describes an apparatus for feeding an elongate article from a floating vessel and into a body of water by using a rotatable cylindrical body. An endless chain of elements are wrapped a number of times around the cylindrical body, and the elements are configured for supporting the elongate article. The number of turns must be adjusted depending on requirements such as load, available space, cost, etc. In a typical laying operation the number of turns are more than 2, preferably between 2 to 5 times, for example 3.75 times. This solution reduce the need for an amount of tensioners significantly by utilizing the well known capstan effect to relieve most of the load from the elongated article. It suffers though from the disadvantage that a dedicated guiding means situated underneath the cylindrical body for guiding the mainly parallel arranged elongated articles in the radial direction during winding. As for VLS this additional arrangement makes the system somewhat complex, maintenance demanding and costly.
In addition, non of the above mentioned publications discloses a solution for handling end termination supports (ETS) for the flow lines/elongated articles that enables a smooth laying operations without manual intervention.
The present applicant has devised and embodied this invention to overcome the shortcomings of the prior art and to obtain further advantages.
The invention makes use of the well-known capstan effect which relates to the hold-force required to counter a load-force when a flexible line is wound around a cylinder (a bollard, a winch or a capstan). Because of the interaction of frictional forces and tension, the tension on a line wrapped around a capstan may be different on either side of the capstan. A small holding force exerted on one side can carry a larger loading force on the other side. This is the principle by which a capstan-type device operates.
The formula which relates the hold-force to the load-force can in most cases be approximated as:T1=T0eμφwhere T1 and T0 represent the outgoing and incoming tensions, respectively, μ is the frictional coefficient between contacting solids such as reel surface and flexible tube, and φ is the contact angle or the total angle swept by all turns of the rope, measured in radians. That is, if the flexible tube or rope performs 3.75 turns around the reel, φ is 3.75·2π=7.5π. For the particular application disclosed herein typical values of μ are in the range from 0.1 to 0.3. Hence, flexible tubes winded 3.75 times around the reel experience an approximate increase in tension or loading force of roughly 10 to 1200 times the incoming tension or holding force.